While whey and casein are found together in milk, when isolated these two proteins have different effects. Whey is higher in some amino acids like leucine and it has a much faster rate of digestion compared to casein. Based on these qualities, whey has been shown in younger subjects to have a greater stimulatory effect on muscle protein synthesis. Older individuals have a blunted response to protein and thus it may be that the benefits of whey are even greater in elderly.

Canadian researchers had elderly subjects perform a single leg resistance exercise protocol and then supplement with either whey or casein immediately after exercise. Both drinks contained 20 grams of protein, but the whey drink had 2.6 grams of naturally occurring leucine compared to 1.6 grams in the casein drink.

Muscle protein synthesis was calculated in the non-exercised leg and exercised leg for 4 hours after exercise. Compared to casein, in the whey group, muscle protein synthesis was 65% greater in the non-exercised leg and 69% greater in the exercised leg.

Thus in elderly men, whey promoted a considerably greater anabolic effect on muscle at rest and after resistance exercise.

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